Download Interpreter Pattern

Interpreter
CS 124
Reference: Gamma et al
(“Gang-of-4”), Design Patterns
Some material taken from:
http://www.dofactory.com/Patterns/PatternInterpreter.aspx
Interpreter
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Intent
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Given a language, define a representation for its grammar
along with an interpreter that uses the representation to
interpret sentences in the language
Motivation
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Interpreting and evaluating expressions governed by a
rules of some language is a common programming
problem
 e.g., arithmetic expressions, regular expressions
This pattern provides a way to define the grammar for the
language, represent sentences, and then interpret those
sentences
Design Solution
Participants
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Client, context, expression
Client typically calls an evaluate method on
the context object
The call, in turn, calls interpret on several
expression objects, and culminates in a
returned result
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The expression objects together represent the
entire sentence, and hence are usually contained
in another object
Examples
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Roman numeral parsing:
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From http://www.dofactory.com/Patterns/PatternInterpreter.aspx
Simple arithmetic expression parsing:
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Reverse polish notation
From
http://en.wikipedia.org/wiki/Interpreter_pattern
Example: parsing
arithmetic expressions
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Client (InterpreterDemo) creates Parser
object using input sentence
Client calls evaluate() on the Parser object
Parser contains a context object (a Stack)
and a list of Expression objects
evaluate() calls interpret(context) on each
Expression object
Desired result is returned via the context
object
Related Patterns
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The expression structure follows the
Composite pattern